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Massive Impact of Observations of the Chameleon-like Behavior of Neutrinos

October 2013

by Simon Mitton

Particle physics is currently going through a period of rapid progress on the puzzle of the origin of mass. On 14 March 2013, news of the tentative confirmation of the Higgs boson, with a mass 250,000 more than the electron, verified that the Higgs field exists. What this means in simple terms is that fermions (quarks and gluons) acquire their mass through interactions with that field. But that’s not the whole story of current research on the masses of elementary particles.

An analysis of highly cited papers in physics, using Thomson Reuters Web of Science, reveals that particle physicists are also very busy on the trail of disappearing neutrinos. The neutrino mass is at least 500,000 times smaller than the electron mass, and 1.25 x 1011 times lighter than the Higgs boson. Measuring its actual value is a key stage in establishing the mass hierarchy of the three flavors of neutrino (electron, muon, and tau). That would extend particle physics beyond the Standard Model, which assigns zero mass to neutrinos.

Figure 1: In the Standard Model of Particle Physics there are six quarks, six leptons, four gauge bosons, and the Higgs boson. In this representation, the three neutrino masses are given as upper limits. [Source: Wikibooks]

Currently it’s an exciting time in neutrino physics. The Web of Science analysis of 41 papers on neutrino oscillations published between 2008 and 2012, as rendered in Figure 2, shows a rising trend for papers published, up from just 4 in 2008 to 16 in 2012. Citations have risen from 53 for the 8 papers published in 2009, to 787 in 2012.

Figure 2: Publication and citation history for selected papers on neutrino oscillations, 2008 to 2012, as displayed in Web of Science

The table below lists ten of the most-cited papers on neutrino disappearance published over the last five years.

Why neutrinos oscillate

The phenomenon of neutrino oscillation, confirmed experimentally in 2001, is the key that unlocked the secrets of neutrino masses. Strange things happen in the quantum world, where neutrinos created with a specific flavor (such as electron neutrinos) but are later found to have changed flavor (by morphing into a muon or tau neutrino). This is not because the electron neutrino suddenly changes to a muon neutrino.

What’s going on can be explained with intermediate level quantum mechanics: the eigenstates—discrete values for fundamental properties of the neutrinos—are in a state of flux: their values are, in effect, probability functions. At the root of this behavior is the Uncertainty Principle: it is impossible to measure the position and momentum of a particle simultaneously. This is fundamental to quantum physics and has nothing to do with the limitations of our technology.

So, to add to the confusion, neutrinos do not have a well-defined mass in classical terms. But there are three “mass eigenstates” and three “flavor eigenstates,” and when a neutrino is detected it will be in one of the three flavors. The mathematical formalism for handling this mix-up involves setting up column vectors for the mass and flavor eigenstates and using a 3 X 3 rotation matrix to handle the probabilities of the oscillations between the electron, muon, and tau states.

The result of this mixing of states is that a neutrino made via the weak interaction will evolve over time to have some probability of being found with another flavor state. In terms of the math, the amplitude of an oscillation probability is given by the so-called mixing angle, θ (theta). The wavelength of the oscillation is set by the square of the mass difference—Dm2—between the initial and final states.

Observing appearance and disappearance

In experimental terms, neutrino physics is currently a search for six extremely small numbers, three mixing angles and three mass differences, corresponding to the three see-saws: electron-muon, muon-tau, and electron-tau.

Nuclear reactors are a copious source of electron antineutrinos—for free. To examine other neutrino types, experimentalists fire protons from an accelerator at a graphite target, which has the advantage that the properties of the emerging
neutrinos can be fine-tuned.

Given that the oscillation is a function of the distance that a neutrino has traveled from its source, the experimental set-ups generally use identical detectors at different distances from the source. And the game is to observe the disappearance of source neutrinos or the appearance of morphed neutrinos in order to understand the physics behind the switch.

The most-cited paper in this Thomson Reuters analysis reported the detection of just six electron neutrinos 295 km downstream from a muon neutrino beam produced by slamming 1.43 x 1020 protons into a graphite target. Six needles in a gigantic haystack! The paper reported that poor statistics in the data meant that zero neutrino mass could not be excluded. But the citation record clearly shows its role as a foundation paper.

In general, appearance experiments are more difficult to run and to interpret than disappearance experiments. Paper #8 is a short report of a search for muon- to-electron oscillation. The evidence for such oscillations dates as far back as 1995, with several searches having poor statistics hinting that the number of electron neutrinos was exceeding that expected from theory. Unfortunately the results from the search described in #8 were also inconclusive.

The mixing angle surprise

So, the research action currently focuses on disappearing neutrinos. The key papers here are #2 and #3. Both experiments study the disappearance of reactor electron antineutrinos over a baseline of about 1 km. They are designed to measure the angle θ13 in the 3 X 3 mixing matrix associated with electron-tau oscillations. At Dava Bay, China (#2), there are six reactors grouped in three pairs, and six detectors of electron neutrinos arranged in three underground experimental halls. Meanwhile, at Chooz in the Ardennes, France, there is a long baseline oscillation detector that counts the neutrinos coming from two nuclear reactors.

Prior to the results in #2 and #3, knowledge of θ13 was an upper limit, and it could have been zero. Double Chooz was the first reactor experiment to get a reliable result (significance >5σ), and—surprise, surprise—it was big! Five months later Daya Bay reported essentially the same result. θ13 was definitely not zero. Physicists were closing in on neutrino masses.

New physics

Neutrino physicists scrambled to organize further experiments and convene workshops in pursuit of the new physics that beckons beyond the Standard Model, and that is why the top papers in our citation analysis are receiving so much attention. New appearance experiments will allow researchers to establish the mass hierarchy of the three neutrinos. The field can also progress searches for CP violation (Charge-Parity symmetry) in neutrino oscillations.

Both goals are highly significant for cosmology, in order to account for the dominance of matter over antimatter in the present Universe. And, more speculatively, neutrinos may yet spring a surprise that is relevant to understanding more about dark matter and dark energy.

Dr. Simon Mitton is a Fellow of St Edmund’s College, University of Cambridge

The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.

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Abstract: “We present cosmological parameter constraints based on the final nine-year Wilkinson Microwave Anisotropy Probe (WMAP) data, in conjunction with a number of additional cosmological data sets. The WMAP data alone, and in combination, continue to be remarkably well fit by a six-parameter Delta CDM model.

The Physics Hot Ten currently has five papers on cosmological results from probes and telescopes in space (#1, #2, #3, #8, #10). The quest to understand the nature of the universe continues with two papers on searches for the Higgs boson (#4 and #7) and one of the hunt for dark-matter candidates (#6).